SECTION - THREE

Welding
SAFETY PRECAUTIONS
Protective clothing is most important in welding.
Fire resistant gauntlet gloves should be won for most welding
operations.
Do not touch the electrode or fix the electrode with bare hand.
Do not touch the welded with bare hand.
piece
Do not lean the
over welding booth or table.
Do not remove the used electrode from the Electrode holder
with bare hand.
Do not use water to extinguish an electric fire.
Do not stand over electric cable.
Do not chip the welded work piece without tongs and goggles.
9. Fumes and gases can be dangerous to health. Keep your head
Out of the fumes. Use enough ventilation.
10. Do live electrical parts
not touch
DEFINITION
Welding is a process of joining similar metals permanently by application of heat with or without
application of pressure and addition of filler material. The result is a continuity of homogeneous material, of the
composition and characteristics of two parts which are being joined together

HISTORICAL BACKGROUND

Welding can trace its historic development back to ancient times. The earliest examples come from the
Bronze Age. During the Iron Age the Egyptians and people in the eastern Mediterranean area learned to
pieces of iron together. Many tools were found which were made approximately during 1000 B.C.
weld
middle ages, the art of blacksmithing was During the
developed and many items of iron were produced which were welded
by hammering. In Late 1800s Gas welding and Cutting was developed. In 1900 Strohmenger introduced a
metal electrode in Great Britain. coated

TYPES OF WELDING
Welding is classified under two broad headings:
(1) Plastic welding or Pressure welding.
(2) Fusion welding or Non-pressure welding.

In theplastic welding or pressure welding, the pieces of metal to be

and then forced together by external joined are heated to a plastic state
pressure. This procedure is used in forge welding, and resistance welding in
which pressure is required.

In the fusion welding or non-pressure welding, the material at the joint is heated to molten state and
allowed to solidify. This includes gas welding, arc a
welding, etc.
GAS WELDING

Gas welding is done by burning a combustible gas with air or

temperature. The purpose of flame is to heat and oxygen in a concentrated flame of nig
melt the metal
parent and filler rod of
of the common materials.
Equipment is
inexpensive, versatile, and
a joint. It can weld mos
repair shops.
serves
adequately in many job and genera
Gas welding is
accomplished by melting the edges or surface to be
molten metal to flow together, thus joined by gas flame and allowing the
forming a solid continuous joint upon cooling. This
suitable for joining metal sheets and plates process is particulaty
having thickness of
additional metal called filler metal is added to the weld in the 2 to 50 mm. With materials thicker than 1
form of welding rod
[Fig 3.1]
 TORCH
TIP
WELDING ROD

INNER -DIRECTION OF WELD

CONE

MOLTEN BASE SOLIDIFIED

METAL METAL WELD METAL

Fig.3.1. Gas Welding Process

Red Hose

Acetylene Torch
Valve

Green Hose

Pressure Gauge
with Regulator
Oxygen
Valve

Acetylene Oxygen
Cylinder Cylinder
Fig.3.2 Gas welding apparatus

Various gas combinations can be used for producing a hot flame for welding metals. Common mixture
of gases are oxygen and acetylene, oxygen and hydrogen, oxygen and other fuel gas, and air and acetylene. The
oxygen-acetylene mixture is used to a much greater extent than the other mixtures and has a prominent place in
the welding industry [Fig 3.2]. The temperature of the oxy-acetylene flame in its hottest region is about 3200° c.

Flame types

The combustion of oxygen and acetylene produces an extremely hot, concentrated flame which may be
adjusted to give various types of flames for different applications. Three distinctly different flame settings are
given in Fig.3.3.
When acetylene is burned in air , it produces a yellow sooty flame which is not enough for welding applications
As the oxygen is turned in surrounded by
on, the
flame immediately changes to a long
white inner area (feather)
a transparent blue envelop. This is called as Carburizing flame. This kind of flame has low temperature
3000°C) due to an excess amount of fuel gas (acety lene). Because of less oxygen, combustion is incomplete and
carbon paricles may be deposited over the weld surface. More oxygen has the effect of reducing he white
feather and increasing the outer, blue envelop. Carburizing flames are used for hardening the surfaces.

55
 ARC WELDING

Covered Electrode Electrode covering (Flux)

Electrode wire ( Filler

metal)
Molten metal D
Gas shield

Molten metal pool Arc colurm

.Slag (Flux)
Weld bead

Heat affected zone

Base metal

Fig 3.4 Arc

Welding Process
Arc welding is the most
an electric arc. The arc column is extensively employed method of joining metal parts. Here the
generated between an anode and the
electric circuit are brought and separated
source of heat is
cathode. When these two
flow through a path of ionized
by a small
distance (2 to 4 mm) such that conductors of an
particles
(gaseous medium) called plasma, an electric the current continues to
gas column acts high resistance conductor that
as a
arc is formed. This
Heat is generated as the ions strike the enables more ions to flow ionized
cathode. from the anode to the cathode.
Hence electrical
Approximately 1 kWh of electricity will create 250 calories energy is converted to heat
of the arc being 6000 to (1000J) of heat energy. The energy.
between which it is struck.
70O0 C. The
temperature of an electric arc, depends temperature at the centre
the upon type of electrodes
The heat of the arc raises the
metal. The electrode metal (in metal temperature of the parent metal which is melted
welding) or welding rod (in carbon arc
arc forming a pool of molten
transferred into the molten metal in the form of globules. The welding) is also melted and is
to fuse and build
up the parent metal surface. deposited metal serves to fili and bond the joint or
remaining One-third is developed near the Two-thirds of theAs heat is developed near the positive pole while the
positive pole will burn away approximately 50negative pole. a result, an
electrode that is connected to the
percent faster than that is connected to the
helpful in obtaining the desired penetration of the base metal. negative pole. This is
ARC WELDING EQUIPMENTS AND TOOLS
The most
commonly used equipment for arc welding consists of the following:
1. AC Transformer DC Generator 2. Electrode 3. Electrode holder
4. Cables. Cable
connectors 5. Ground clamps
7. Wire brush 6. Chipping Hammer
8. Safety goggles 9. Hand gloves
10. Aprons, sleeves. etc

MARGLUE TTE

A.C. ARC WELDLR

Fig 3.5 AC Transformer

57
 A Transformer arc welding, each havin its
used for eiectric
current are drticula
Both direct current and alternating obtained irom generators driven
DC welding is
usually by
applicatuons; in
either is suitable.
s o m e cases
combustion engines.
For AC welding supply. transform electric
mers are
motor or if no electricity
is available, by internal is a v a i l a b i e . It has to
to as step down the
used for aimost all arc welding
where electricity supply usual
predominantly circuit welding
voltage (50-90 volts).
voltage (200-400 volts) to the normal open
supply

ELECTRODES welding are classified in to

two categories
Electrodes (Filler Rods) for arc

either bare or c o a t e d .
Consumable electrodes electrodes may be type.
metals. The consumable tlux. During welding a are
h e s e are made of different coating of
metals which do not have any
made of various the molten metal is oxidized
or plain electrodes are the oxvgen of
the surrounding
air. H e n c e .
decreases the strength
The
the bare electrode is exposed to metal also,
which ultimate ly
contains the oxidized where quality of weld ie h e
molten pool after solidifying like minor repair so
electrodes have limited applications
joint. Hence bare
important

Ccating

Wire

Fig 3.6 Coated Electrode

Fig 3.7 Electrode holder

Electrodes coated with fux material, such as chalk, aluminium, ferro-manganese starch, alloying and binding
materials, are called flux coated electrodes ( Fig 3.6). Electrodes are generally specified by diameter and length
like 2.5 x 350mm

The flux coating serves the following purposes:

(i) It facilitates the stability of the arc.

ii) It protects the molten metal from the atmospheric oxygen and nitrogen by producing gaseous
shield around the arc and the molten pool.
(ii) It provides the formation of slag which floats the top of molten
on
pool and protects the
welded seam from oxidation and rapid
cooling.
Non-consumable Electrodes
These are made of carbon, graphite or
tungsten which do not get consumed during welding process.
Electrodes are
generally available up to 12 mm diameter and 450 mm
long.
ELECTRODE HOLDER
Electrode holder is a device used to secure electrode. Most electrode holders have
jaws which enable the electrode to be held at various grooves cut in to
uic
angles forease in
manipulation [Fig.3.7.]
GROUND CLAMP
The ground clamp completes the circuit between the
generally fastened to the metal being welded either with a electrode and the welding machine. This
clamp, a or bolt, some other means.

58
CHIPPING HAMMER AND WIRE BRUSH
The chipping hammer is chisel
The wire brush, which removes
shaped and is pointed on one
small end to aid in the
removal of slag.
embedded in wood. particulates of slag, is generally made of stiff steel wire

Point

Chisel
Fig 3.8 Ground clamp
Fig 3.9 (a) Chipping Hammer (b) Wire Brush
SAFETY ACCESSORIES
Provisions for the safety of the operator are
most
nrotection device will stop the radiation that is important to ensure workable situations. A face
face shield are commonly used in arc welding. prevalent in arc welding. Helmet
face shield and the hand held
Long gloves protect the hands from
chipping operation. minor burns during the

Fig 3.10 Face shield

The gloves should be worn all the time during welding. The work clothes that should
welder consist be worn by the
mainly of long sleeved cotton shirts and long pants. Shoes should be high topped.

WELDING TECIINIQUE

The termwelding technique implies the direction in which the heat is concentrated during welding. The
heat may be concentrated either in the weld bead or ahead of the weld bead during the
process of welding.
Depending upon whether the heat is concentrated on the weld bead or ahead of the weld bead, the welding
technique is classified as,

1. Forehand welding 2. Backhand welding

Forehand Welding
In the forehand welding, shown in fig 3.12, the torch points in the same direction in which the welding
Deing done so that the heat is not flowing in to the metal as much as it could. The forehand welding is used for
relatively thin parts.

59
Backhand Welding
3.11 the torch is pointing
in the direction opposite to
direction opnoe:

welding, shown in fig. into the metal, so thats hat in

In the backhand
which the welding
is being done. n this technique, the heat is
concentrated
nicker pans
canbe welded successfully.

Torch tip
Torch tip

Bead Bead
Bead

Base metal
Base metal
Base metal
Direction of torch travel
Direction of torch travel
A weid bead Backhand welding
Forehand welding

Fig3.11 Welding Techniques

TYPES OF WELDING JOINTS

The relative positions of the two pieces being joined determine the type of joint. Five basic tvpes af
joints are used in fusion welding. These are; Butt, Lap, Tee, Corner, and Edge joints. There are also several
variations of each of these joints [Fig 3.12]. The nature of the joint depends upon the kind and size of material
the process, and the strength required. A joint is selected in each case to fulfill requirements at lowest cost.

The Butt joint is used to join the ends or edges of two plates or surfaces located approximatelyinthe
same plane with each other. On thickness from 2 to 5 mm, the open square butt should be selected, but thicknes
upwards of smm, joints with edge preparation on one or both sides may be recommended.

The Lap joint, as the name implies, is used to join two over
is welded to the surface of the other. Common
lapping plates so that the edge of each plate
lap joints are single lap and double lap. The
does not develop full strength, but it is
preferTed to the butt joint for some
single-welded lap
may be employed for thickness under 3mm. applications.
The lap joint, however,

The Tee-joint is used to weld two

each other. Plates or surfaces should have plates
or sections whose
surfaces are at approximately right angles to
suitable up to 3mm and is good fit-up in order to ensure uniform penetration
widely employed in thin walled structures. and fusion. This i5
The Corner joint is used to
join the edges of two sheets or
approximately 90° to each other. It is
common in the
plates whose surfaces are at an angie 0
items. Welding can be done on one construction of boxes, tanks,
suitable for both light and heavy
or both
sides, depending on the frames, and other simila
gauges. position and type of corner joint used. I nis o
corner is
Lap-joints, T-joints, and corner joints are the fillet
known as filleting. Fillet weld connections
welded joints are favoured
by designers in thegenerally
load conditions used. The rounding of a
permit. There are three types of fillet weld: interest of fabrication
convex, flash and concave. cost
The edge
joint consists of joining two
sheet metal work. The two parallel plates by means of a
metal. In heavy plates, whereedges can be easily and quickly melted weld. This is oftern us in
position of welding is marked beveling
the edges is
done to get down, eliminating thejointneed for Tiller
with a cross
(X) in each case deeper penetration, some filler rod is any The
neeuc

60
 (a) Butt joint
(b) Lap joint

(e) T-joint

(c) Coner joint

(d) Corner joint
Fig 3.12 Types of welding joints
EDGE PREPARATION

To obtain sound welds,

good edge preparation is particularly essential. consisting
the edges, and carefuly cleaning the faces to be in suitably beveling
welded from dust, sand,
grit, oil and grease.
Different edge preparatior is
particularly used in fusion welding processes for welding butt joints
(e) double-U. The preparation of edges depends upon are
(a) square (b) single-V (c) doubie-V (d) single-U
thickness of metal being welded. the

Square butt weld may be used for thickness of from 3 to 5mm. Before
about 3mm apart. welding. the edges are spaced
Single-V butt welds are frequently used for metal over
8mm and up to about 16mm thick. The
forming the joint are beveled to form an included angle of 70° to 90", edges
be used. depending upon the welding technique to

Double-V butt welds are used on metals over 16mm thick and where
sides of the plate. welding can be performed on both

Single-U and double-U butt welds are used on metals over 20mm thick. They are most satisfactory and
require less filler rod; but they are difficult to prepare.

70-90
Root

Square Single

Double- Single - U
Doubie-U

Fig 3.13 Types of Edge preparation

61